1. Field of the Invention
The present invention relates to a printhead substrate, printhead, head cartridge, and printing apparatus. Particularly, the present invention relates to a printhead substrate which is used to print according to an inkjet method and has a circuit for driving a heater by supplying a predetermined current to it according to a constant electric current method, a printhead, a head cartridge, and a printing apparatus.
2. Description of the Related Art
There has conventionally been known an inkjet printhead which prints by generating thermal energy from heaters arranged in the nozzles of a printhead, bubbling ink near the heaters using the thermal energy, and discharging ink from the nozzle by the bubbles. The inkjet printhead will be simply referred to as a printhead.
Recently, inkjet printing apparatuses using the printheads are required to achieve higher speeds and higher resolutions. To meet these requirements, the printhead integrates a larger number of nozzles at higher density. When driving heaters in the printhead, as many heaters as possible need to be simultaneously driven at high speed in terms of the print speed.
In general, many heaters and their driving circuits are formed on a single semiconductor substrate (this substrate will be called a head substrate). The heater driving circuit is formed by a MOS semiconductor process which can form smaller devices at higher density by a simpler manufacturing process at lower cost than by a conventional bipolar semiconductor process.
As a new heater driving method coping with high-speed printing and the MOS manufacturing process, the United States Patent Application Publication Nos. 2005/0212857 and 2005/0206685 propose heater driving methods using a predetermined current.
FIG. 13 is a circuit diagram showing the arrangement of the heater driving circuit of a printhead proposed in the United States Patent Application Publication No. 2005/0206685.
As is apparent from FIG. 13, the heater driving circuit includes a reference voltage circuit 105, voltage-to-current conversion circuit 104, and current source block 106. The current source block 106 is formed from m heater groups each including x heaters. Although not shown, one printhead has n current source blocks. Hence, one printhead has a total number of (x×m×n) heaters.
The reference voltage circuit 105 generates a reference voltage Vref serving as a reference for the voltage-to-current conversion circuit 104. The voltage-to-current conversion circuit 104 converts a voltage into a current on the basis of the reference voltage Vref from the reference voltage circuit 105, generating a reference current Iref from the reference voltage Vref.
Based on the reference current Iref generated by the voltage-to-current conversion circuit 104, a reference current generation circuit (not shown) generates a plurality of reference currents proportional to the reference current Iref. These reference currents are supplied to the n current source blocks, respectively.
By using a corresponding one of reference currents IR1 to IRn, constant electric current sources 1031 to 103m in each of the n current source blocks output constant electric currents Ih1 to Ihm proportional to the reference current supplied to the constant electric current sources.
As shown in FIG. 13, the current source block 106 includes (x×m) heaters, switching elements 102 as many as the heaters, and constant electric current sources 1031 to 103m of m groups. In FIG. 13, in order to individually refer to the (x×m) switching elements 102, a suffix “ij” (i=1 to m, j=1 to x) is added to the reference numeral. When referring to the switching elements 102 as a whole, the suffix will be omitted.
The switching element 102 controls short-circuit and open-circuit of a current between terminals in accordance with a control signal from the control circuit of the printing apparatus main body. (x×m) heaters 101 and the switching elements 102 belong to m groups each including x heaters 101 and x switching elements 102. In these groups, the heaters 10111 to 101mx and the switching elements 10211 to 102mx for controlling driving of the respective heaters are series-connected. In each group, power supply terminals are commonly connected to a power supply line 110, and ground terminals are commonly connected to a GND line 111 via constant electric current sources. In FIG. 13, in order to individually refer to the m groups, a suffix is added like 106-1, 106-2, . . . , 106-m. When referring to the m groups as a whole, they will be denoted by the reference numeral “106”, and the suffix will be omitted.
The output terminals of the constant electric current sources 1031 to 103m provided in correspondence with the m groups 106 are connected to the common connection terminals of the corresponding groups 106 in which the heaters 101 and switching elements 102 are series-connected. Driving control of a current to the heaters is executed by turning on/off the switching elements 102 in each group in accordance with a control signal. Output currents Ih1 to Ihm from the constant electric current sources 1031 to 103m provided in accordance with the respective groups are supplied to desired heaters.
In an actual printhead, a plurality of (n) current source blocks 106 having the same arrangement are provided, and the heater driving operation of each current source block 106 is the same as that described above. The n current source blocks 106 perform the same operation to drive any desired ones of (x×m×n) heaters and generate heat. There is known a control circuit in which a reference current generator Iref is connected to a switch and selects a constant electric current to be supplied to a heater, as disclosed in Japanese Patent Laid-Open No. 2000-246900. There is also known a printhead which changes the value of a reference current generated using a print data signal input from the outside of the head substrate, as disclosed in the United States Patent Application Publication No. 2007/0211095.
However, the conventional techniques suffer the following problems.
(1) Unstable Discharge and Discharge Failure in Low-Temperature Environment
When the temperature in an environment where the printing apparatus is installed is excessively low (e.g., 10° C. or less), the ink viscosity increases, the discharge becomes unstable, and in the worst case, a discharge failure occurs.
(2) Difference in Rise Characteristic of Head Temperature Depending on Print Duty
The temperature rise characteristic of the head substrate differs between printing of a high-density image and that of a low-density image, and the ink discharge state changes. As a result, dot formation for a high-density image and that for a low-density image differ from each other, degrading the image quality.
(3) Difference in Rise of Head Temperature Derived from Difference in Set Current Value between Head Substrates
The electrical resistance of a printing element (heater) varies owing to manufacturing variations of head substrates. To generate a predetermined thermal energy by the printing element, the value of a current flowing through the printing element needs to be changed. For this purpose, a reference current (to be referred to as a set current value hereinafter) serving as the reference of a current to be supplied to the printing element is changed for each printhead. The heat generation amount of a reference current generation circuit differs between head substrates, and the temperature rise differs between them. When the printhead is changed, density unevenness or the like appears in a printed image, changing tonality.